The present invention pertains to communication systems, and more precisely to communication systems carrying multimedia services to handheld terminals over digital video broadcasting (DVB) networks.
Internet Protocol DataCasting (IPDC) combines IP-based data transmission with digital broadcasting to enable distributing multimedia content to a large group of users.
In particular, DVB-H (DVB Handheld) standard compliant solutions allow for the delivery of multimedia content, such as IP-TV, to mobile devices.
The following references from the European Telecommunication Standard Institute (ETSI) notably apply for the DVB-H system:                ETSI EN 302 304: <<Digital Video Broadcasting (DVB); Transmission System for Handheld Terminals (DVB-H)>>;        ETSI TR 102 377: <<DVB-H Implementation Guidelines>>;        as well as the various references cited in these documents.        
A typical DVB-H network is a Multiple Frequency Network (MFN) comprising multiple coverage areas (or cells), each of these areas being of the Single Frequency Network (SFN) type.
FIG. 1 represents such a DVB-H network.
Three cells C1, C2, C3 are shown which each use several transmitters T. All the transmitters within a cell receive the common to the cell DVB Transport stream through a regular distribution network DN, and modulate and broadcast this Transport stream on the same one frequency.
The network is said to be MFN in that neighbouring cells C1, C2, C3 use different transmission frequency channels F1, F2, F3. In particular, the use of different frequency channels in neighbouring cells gives the possibility to run local content in each cell. For instance, local TV programs can be broadcasted in each cell.
Associated with the transmitters are repeaters R that may be used to fill in shadows in the reception pattern, and to improve coverage on critical areas where reception performance is insufficient. A DVB-H repeater operates at RF level and receives a DVB-H emission from the air at a certain frequency (i.e. the transmission frequency used within the cell where the repeater is located), amplifies the received emission and retransmits it in the same frequency.
An IP-backbone is responsible for delivering the multimedia content (IP datagrams) to the different cells C1, C2, C3.
In each cell, an IP Encapsulator (IPE) is commonly used which acts as an interface between a broadcast source (via the IP backbone) and the distribution network DN distributing the content to the different transmitters within the cell.
The IP data to be broadcasted are inserted into PES (Program Elementary Stream) packets mapped into MPEG-2 (Movie Pictures Expert Group) TS (Transport Stream) packets, as defined in MPEG-2 Systems ISO/IEC 13818-1.
The IPE thus prepares the delivery of MPEG-2 TS from the IP datagrams it receives from the IP-backbone.
The IPE takes the responsibility for encapsulating the incoming IP-datagrams into MPE (MultiProtocol Encapsulation) sections, these MPE sections being afterwards segmented to fit in MPEG-TS packets.
The IPE also adds the required PSI/SI (Program Specific Information/Service Information) signalling data.
The IPE also includes time slicing technologies for sending data in bursts, along with MPE timing information which allows for indicating to the receiver when to expect the next burst (the relative amount of time from the beginning of this MPE frame to the beginning of the next burst being indicated within a burst in the header of each MPE frame). This enables the handset to shut down the receiver between bursts, thereby minimizing power consumption and preserving battery life.
Other time-slicing signalling information, such as burst duration, are included in the time_slice_fec_identifier_descriptor in the INT (IP/MAC Notification Table). Some of this information is also sent within Transmission Parameters Signaling (TPS) bits that are transported by dedicated carriers (TPS Pilots) in the COFDM (Coded Orthogonal Frequency Division Multiplexing) signal so as to be more quickly and easily available by the receivers (which thus do not need to decode MPEG2 and PSI/SI info).
The IPE can also introduce an additional level of correction at the MPE layer by generating MPE-FEC (MultiProtocol Encapsulation-Forward Error Correction) frames. The objective of the MPE-FEC is to improve the carrier to noise ratio (C/N) and Doppler performance in mobile channels and to improve the tolerance to impulse interference.
A SFN adapter is responsible for forming time-stamped groups of frames (named megaframes) and sending them on a distribution network to the transmitters of the cell, thus allowing for the modulators associated with the transmitters to be accurately synchronized and to deliver the same bits in the same COFDM carriers at the same time. All the transmitted signals are thus identical which allows for the simultaneity of reception of the information by all the listening users.
In particular, GPS receivers may be used to provide a time reference for SFN synchronization.
A megaframe (see ETSI TS 101 191) consists in a group of n TS-packets or RS-packets (TS-packets with OFDM Reed Solomon coding information added), where n is an integer number, which depends upon the number s of RS-packets per OFDM superframe in the DVB-T transmission mode which is used (see EN 300 744 sub clause 4.7). In the 8K mode n=s×2, in the 4K mode n=s×4, in the 2K mode n=s×8. The first packet of a megaframe has an inverted sync byte.
A megaframe comprises a synchronization mark known as a megaframe Initialization Packet (MIP). A MIP is an MPEG2 packet with a dedicated (0x15) PID (Packet IDentifier) value. The MIP of a megaframe of index M allows to uniquely identifying the starting point of the megaframe of index M+1.
A MIP carries information to transmitters concerning the position of the first packet of the next megaframe in the Transport Stream (expressed in number of MPEG-2 TS-packets), timing information (with a 100 ns accuracy) indicating to the modulator when the modulation of next megaframe must be started, configuration values for modulation and transmission parameters (mode, guard interval, cell identifier, etc.) that the modulators must send in the TPS carriers, and finally, thanks to the function parameters, specific values such as time and frequency offsets, transmission power, cell identifier, . . . for a given transmitter or all the transmitters.
Timing information included in MIP are mainly the synchronization_time_stamp (STS) and the maximum_delay, both expressed in 100 ns units.
STS contains the time difference between the latest pulse of the “one-pulse-per-second” reference that precedes the start of the megaframe M+1 and the actual start (in the SFN adapter) of this megaframe.
The maximum_delay field contains the time difference between the time of emission of the start of megaframe M+1 by the antennas of all the transmitters in the cell and the start of megaframe M+1 at the SFN adapter. The value of maximum_delay shall be larger that the sum of the longest delay in the distribution network DN and the delays at the modulators, power transmitters and antenna feeders. The maximum value of maximum_delay is 1 second. A transmitter receiving the start of a megaframe at Trec (expressed in 100 ns units relative to the current pulse of the one pps reference) delays this megaframe by Tdelay=(STS+maximum_delay−Trec) modulo 107.
SFN Adapters are usually placed in each cell behind an IPE or a Multiplexer, but the SFN Adapters capabilities can be included in the IPE or in the Multiplexer itself.
However other network topologies can be contemplated. For instance, the IP datagrams, instead on being encapsulated locally at a local IPE, may be encapsulated centrally and distributed within a centrally produced transport stream to the sites where the final transport stream is produced and broadcasted.
Such a centrally encapsulated stream could also be time-sliced, both the encapsulation and the time-slicing being then centrally performed, using a “central IPE”. The task of the SFN adapter for the insertion of MIP synchronization marks within the transport streams could also be implemented centrally.
In this case, there is no need for a local IPE in each cell. Furthermore, the intra-cell distribution network (linking a local IPE to the various transmitters) is also not mandatory, as the centrally encapsulated and time-sliced stream can be delivered directly from the central IPE to the transmitters of the different cells, for instance via a satellite link.
It has to be noted that such a satellite delivery of a centrally produced content should be used in the future, for instance to respond to complexity, cost and coverage problems. Indeed DVB-H requires a denser transmitters network than DVB-T, and thus an existing DVB-T intra-cell distribution network has to be densified in order to answer DVB-H constraints. Of course, such a densification is complex and expensive. Furthermore, the deployment of DVB-H should be carried out by private companies, without public utility obligation. There is thus a risk that these companies concentrate on the zones with high population density, possibly leaving uncovered zones with low population density. In view of the above mentioned problems, satellite delivery is appreciable because of its ease and speed of implementation and of its low capital costs.
But a centrally generated DVB-H TS is common to the entire MFN network and the same programs are thus broadcasted in all the cells.
And the central IPE solution does not take advantage of the DVB-H network topology, and notably the fact that this network covers various geographical areas (the SFN cells).
An objective of the present invention is to take advantage of the DVB-H multiple SFN cells network topology despite the fact that the TS is centrally produced, and aims notably at providing a technique which would allow, starting from a centrally produced TS, for particular local TSs to be produced and synchronously distributed in each particular SFN cell.
For this purpose, and according to a first aspect, the invention proposes a method for processing a transport stream received as an input transport stream in a processing device, the transport stream comprising a plurality of elementary streams, each elementary stream (ES) being a set of transport stream packets having the same Packet IDentifier (PID), at least one of these elementary streams being time-sliced and sent in bursts, timing information indicating within a burst the time to the beginning of the next burst, characterized in that it comprises the steps of:                applying a filtering operation to the input transport stream so as to filter out from the input transport stream part or all of one or more time-sliced elementary streams;        modifying the bursts scheduling of the input transport stream so as to generate a DVB-H compliant output transport stream from the filtered input transport stream.        
According to a second aspect, the invention proposes a device for processing a transport stream comprising at least one time-sliced elementary streams, characterized in that it comprises means for performing the method according to the first aspect of the invention.